Amplifier



June 1939 H. MAYER 2,162,267

AMPLIFIER Filed May 14, 1936 Az Y INVENTOR HANS MAYER ATTORNEY PatentedJune 13, 1939 UNITED STATS PATENT OFFICE 2,162,267 AMPLIFIER ApplicationMay 14, 1936, Serial No. 79,631 In Germany July 10, 1935 3 Claims.

It is known that non-linear distortions appearing in tubes especially inend stages of amplifier circuits can be eliminated by excessive matchingof the outer load resistance i. e. by choosing the latter high ascompared with the inner resistance of the respective tube. The linearityis produced since at very high values of the outer resistance, thelinear current becomes very small and due to the fact that thenon-linear electro-motive forces controlled by the linear current meetwith a high resistance.

If the linearity obtained by excessive matching is to be applied totubes connected in parallel or in push-pull, an extremely exact equalityof the tubes operating in pairs is required, so as to avoid a decreasein the effect of the excessive matching due to compensating currentspassing between the two tubes.

It is accordingly an object of this invention to prevent distortion inamplifiers connected either in parallel or push-pull.

For a better understanding of the invention reference is made to theaccompanying drawing in which,

Fig. l is a schematic circuit diagram of two amplifiers connected inparallel to a common load resistance,

Fig. 2 is a similar diagram with the amplifiers connected in push-pull,

Fig. 3 is a circuit diagram showing an embodiment of the invention asapplied to parallel connected tubes,

Fig. 4 is a circuit diagram showing the invention as applied to twotubes connected in pushpull, and r Fig. 5 is a circuit diagram showingthe invention as applied to two pairs of tubes connected in push-pull,each pair being connected in parallel.

In Figure 1 there is shown schematically the parallel connection of twotubes loaded by the outer load resistor R. The tubes whose internalresistances are designated by R1 and R2 respectively, produce theelectro-motive forces -U and U respectively. The factor F, designatesthe amplification factor; The potential U is the voltage applied to theinput circuits of the tubes and which is to be amplified.

In case the amplification factors 1.1 and ,uz are not equal it will notbe possible even in case of an infinitely high outer resistor R to causedisappearance of the linear current in the tubes. In the circuit formedby the two tubes a compensating current I flows, whose value is given bythe following equation:

I: U m 2 The tube with the internal resistance R1 hence, does notoperate with the very high outer load resistor but operates with aresistor R'o having the value:

In a corresponding manner the value of the load resistance Ru of thetube having the internal re- An effective excessive matching is thusprevented.

It is difficult to obtain for both tubes the same amplification factorand furthermore in all probability even where initially equal,irregularities will occur during operation or due to an exchange oftubes. In order to obtain, despite these conditions, a suificientlinearity by means of excessive matching the invention provides means bywhich the unbalanced current produced by irregularities in the tubesoperating in pairs will be reduced. This may be accomplished in tubesconnected in parallel by means of a differentia choke for instance,which is placed in series with the two tubes. In push-pull stages thesame effect can be achieved with a choke coil inserted in the commonplate circuit of the two tubes.

Several circuits embodying the invention are represented in Figures 3 to5. Figure 3 shows the parallel connection of two tubes with the internalresistances R1 and B2. In series with the plate circuits of the twotubes there is placed the differential choke coil D having the outerload resistance R. connected to its electrical center. Owing to thedifferential action, the choke does not influence the currents passingthrough the outer resistance R, but it increases the im- UGO pedance ofthe circuit formed by the two tubes R1 and R2. Consequently, thecompensating or circulating current will be decreased, thus obtainingexcessive matching and linearity.

: Figure 4 shows the application of the invention to a push-pullcircuit. The choke D is inserted in the common part of both platecircuits, i. e. between the cathodes of the tubes and the center pointof the primary winding 20 of the output transformer 2|. This arrangementassures the desired excessive matching even in case of unequalamplification factors of the tubes. The choke does not influence theamplification factor and the supplied A. C. energy since the amplifiedcurrents pass through the choke with equal intensity, but in oppositedirections thus not producing voltage loss therein. In this case theload resistor R is shown in the secondary circuit.

Figure 5 shows the end or power stage of an amplifier in which push-pulloperated pairs of tubes are connected in parallel. In this case it isnecessary to prevent the compensating currents between the tubes placedin parallel as well as between the groups operating in push-pull. Thisis carried out in accordance with the invention by interposed chokecoils D, D1 and D2. As in 'the case of Figure .2, the choke coils D1 andD2 are differential chokes.

The invention is not only applicable to amplifier tubes, but in allcases in which the effect of linearity of an excessive matching incircuit elements operating in parallel or in push-pull is to be assuredeven where the electrical properties of these circuit elements differfrom each other.

Having described my invention, what I claim as novel and desire tosecure by Letters Patent is:

1. An amplifier comprising two vacuum tubes, adifferential choke coilhaving itsends connected to the plates of said tubes, and a circuitconnecting the electrical center of said choke coil and said tubecathodes and including an impedance having a value higher than the plateresistance of either of said tubes.

2. An amplifier comprising the combination of two electron dischargetubes each having a cathode, grid and plate, a direct connection betweensaid grids, a 'difierential choke coil having its ends connecteddirectly to said plates and having a high impedance to currents of thefrequency "toibe amplified, and a circuit connecting the electricalcenter of said choke coil and said tube cathodes and including aresistance having a value higher than the plate resistance of one of:said tubes, whereby the current passing through said resistance varieslinearly with the voltage applied to said grids when the amplificationfactors of said tubes are of somewhat different values.

3. A .power amplifying stage comprising the combination of a first pairof vacuum tubes, a second pair of vacuum tubes, each of said tubeshaving a cathode, control grid and plate, a-direct current connectionbetween said tube cathodes, an input transformer having one 'end of itssecondary winding connected to the "grids of 'said first pair of tubesand its other end 'tothe grids of the second pair oftubes, 'a circuitconnecting the midpoint of said secondary winding to said tube cathodes,a differential choke 'coil connecting the plates of the first pair oftubes, a'second differential choke coil connecting the plates of thesecond pair of tubes, a divided output impedance, direct currentconnections between the ends of said impedance and the midpoints of saidchoke coils and a circuit including the series connection of a source ofpositive potential and a high imductance connected between the midpointof said output impedance and said tube cathodes.

HANS MAYER.

